Introduction. The aim of this prospective study was to estimate whether the presence of endometrial fluid detected by transvaginal ultrasound investigation was a marker for the pathological changes of the endometrium in postmenopausal women. Material and methods. 128 postmenopausal women with uterine bleeding and 29 asymptomatic postmenopausal women underwent transvaginal ultrasound investigation, curettage and histopathological investigation of the curettage specimens. Results. There were significantly more asymptomatic women with endometrial fluid collection was found (41,4%) than those with uterine bleeding (7,8%) (p<0,001). We found 4 cases of carcinoma of the endometrium, 4 with hyperplasia and 1 with polyp of the endometrium in women with uterine bleeding and endometrial fluid collection. In the asymptomatic group of women we found 1 case with polyp and 1 with carcinoma of the endometrium. In the presence of endometrial fluid collection the least thickness of the endometrium measured by transvaginal ultrasound was 12 mm in postmenopausal women with carcinoma of the endometrium, 7 mm in women with hyperplasia and 5 mm with polyp. No pathological changes were found below the above values. Conclusion. The presence of endometrial fluid detected by transvaginal ultrasonography is a good marker for pathological changes of the endometrium in postmenopausal women if the endometrial thickness is greater than 4 mm. If the endometrial thickness is 4 mm or less, the presence of endometrial fluid is not an indication for further invasive investigation of endometrial cavity, but we must eliminate possible presence of ednexal or cervical malignant disease in some patients